The disclosure of Japanese patent Application No. 2000-033656 filed on Feb. 10, 2000 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to a traction control apparatus disposed in a four-wheel drive vehicle to control driving forces generated between four driven wheels and a road surface, and to a method of traction control.
2. Description of Related Art
Japanese Patent Application Laid-Open No. 10-217932 discloses one example of a traction control apparatus to be installed in a four-wheel drive car. The traction control, apparatus disclosed therein controls if there is generated a difference in rotational speeds of driven wheels, braking forces applied to the respective driven wheels such that the rotational speed difference is reduced. This makes it possible to achieve substantially the same effect as in the case where a differential limiting mechanism is added to a differential.
However, the braking forces applied to the respective driven wheels are controlled based on the difference in the rotational speeds of the driven wheels. Thus, for example, if the inner ones of the turning wheels have developed a tendency to slip and increased in rotational speed, the braking forces applied to the inner ones of the turning wheels may increase, and the braking forces applied to the outer ones of the turning wheels may decrease. In such a case, the controllability of the vehicle behavior may deteriorate. If all the four driven wheels have developed a tendency to slip-to the same extent, there may be no difference generated among the rotational speeds of the driven wheels. This makes it impossible to set braking forces suited for the respective driven wheels. Also in this case, the controllability of the vehicle behavior may deteriorate.
It is one object of the invention to provide a traction control apparatus and method suited for a four-wheel drive vehicle and capable of controlling the vehicle behavior in a stabilizing direction even if driven wheels have developed a tendency to slip.
A traction control apparatus for a four-wheel drive vehicle according to a first aspect of the invention is disposed in a four-wheel drive vehicle in which driving torques of a drive source are distributed to front-left and front-right wheels and to rear-left and rear-right wheels, and controls driving forces generated between the respective wheels and a road surface. The traction control apparatus includes brake devices disposed in the respective wheels, which generate predetermined braking forces to brake rotation of the wheels, and a controller individually controlling the braking forces generated in the respective brake devices. In this traction control apparatus, the controller simultaneously generates an equal braking force in the brake devices corresponding to the rear-left and rear-right wheels if it is judged that a critical driving force applied to at least one of the rear-left and rear-right wheels is small.
The critical driving force is a driving force that could be generated between the respective driven wheels and the road surface with an unlimited differential among the driven wheels and corresponds in magnitude to the friction coefficient of the road surface and the grounding loads applied to the driven-wheels.
A driven wheel with a small critical driving force may develop a tendency to idle-rotate (spin) when the vehicle is started or accelerated. If at least one of the rear wheels has developed a tendency to idle-rotate the grip degree of a tire mounted to the idle-rotating wheel decreases, and the grip balance between the rear-left and rear-right wheels is lost. This may cause a deterioration of the controllability of the vehicle behavior. Thus, in such a case, the controller simultaneously generates an equal braking force in the rear-left and rear-right wheels, and the brake devices absorb the driving torques distributed to the rear-left and rear-right wheels to the same extent. Thereby the idle-rotating tendency on the side of the rear wheels is weakened, and the grip balance tends to be recovered. At the same time, the driving forces applied to the front wheels become greater than those applied to the rear wheels, so that the vehicle body tends to be pulled forward by the front wheels. Due to such an operation, the behavior of the vehicle body can be shifted in a stabilizing direction.
In the aforementioned aspect, the controller may set the braking forces if it is judged that at least one of the rear-left and rear-right wheels tends to slip, in reference to the greater one of slip amounts of the rear-left and ear-right wheels.
If at least one of the rear wheels has developed a tendency of accelerated slip, the braking forces are set so as to curb the slip, in reference to the greater one of the slip amounts generated on the side of the rear wheels. Therefore, the braking forces can be suitably set in accordance with the slip state, and the vehicle behavior can be stabilized in a suitable manner.
In the aforementioned aspect, the controller may simultaneously generate an equal braking force in the brake devices corresponding to the rear-left and rear-right wheels if it is judged that all four wheels tend to slip.
If all four wheels have developed a tendency to slip, it becomes difficult to detect slip amounts with precision. In the case where all four wheels tend to slip, the braking forces to be generated by the brake devices are set by the controller according to the setting method characteristic of such a case. For example, the braking forces to be applied are determined in advance. In this case, since the equal braking force is applied to the rear-left and rear-right wheels, the vehicle behavior can be shifted in a stabilizing direction without being adversely affected.